Aircraft engines are typically spaced from the fuselage and, therefore, are housed in a nacelle. A typical nacelle is constructed as an aerodynamic housing having a forward portion, commonly referred to as a nose cowl, which defines an inlet into the nacelle. A ring-like structure, commonly referred to as a lipskin, is typically connected to the nose cowl of the nacelle.
Aircraft lipskins are commonly manufactured from aluminum alloys. The aluminum alloy starting material is typically received from an aluminum supplier in plate form and in an annealed state. Then, the aluminum alloy plate is cut into a blank having the desired silhouette, and the blank is then formed into the desired lipskin shape, such as by die-stamping the blank in a press or by spin-forming the blank. After heat treating, final sizing and aging, the surface of the lipskin is typically machined to the desired surface finish and the lipskin is chemically treated and anodized to yield a finished part.
When certain aluminum alloys, such as 2219 aluminum alloy, are used to form lipskins, discoloration is often visible in the final anodized part. For example, the discoloration can appear as visible lines of discoloration on the surface of the part. The discoloration typically presents itself after the forming (e.g., spin-forming) step, but becomes much more acute after anodizing.
Attempts have been made to obscure such surface discoloration, such as by applying a rough, non-directional surface finish prior to anodizing. For example, a surface roughness (Ra) of 125 microinches has been used to obscure such discoloration. However, such a high surface roughness generally will not satisfy stringent surface quality requirements aimed at improving aerodynamic performance.
Accordingly, those skilled in the art continue with research and development efforts if the field of aluminum alloy forming.